Additive manufacturing of polishing pads

ABSTRACT

Interpenetrating polymer networks (IPNs) for a forming polishing pad for a semiconductor fabrication operation are disclosed. Techniques for forming the polishing pads are provided. In an exemplary embodiment, a polishing pad includes an interpenetrating polymer network formed from a free-radically polymerized material and a cationically polymerized material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/897,184, filed Jun. 9, 2020, the contents of which are incorporatedby reference.

TECHNICAL FIELD

This disclosure relates to polishing pads utilized in chemicalmechanical polishing.

BACKGROUND

An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive, or insulativelayers on a silicon wafer. A variety of fabrication techniques employplanarization of a layer on the substrate. For example, for certainapplications, e.g., polishing of a metal layer to form vias, plugs, andlines in the trenches of a patterned layer, an overlying layer isplanarized until the top surface of a patterned layer is exposed. Inother applications, e.g., planarization of a dielectric layer forphotolithography, an overlying layer is polished until a desiredthickness remains over the underlying layer.

Chemical mechanical polishing (CMP) is one accepted technique ofplanarization. In application, this planarization technique may mountthe substrate on a carrier head. The exposed surface of the substrate istypically placed against a rotating polishing pad. The carrier headprovides a controllable load on the substrate to push the substrateagainst the polishing pad. A polishing liquid, such as slurry withabrasive particles, may be supplied to the surface of the polishing pad.One objective of a chemical mechanical polishing may be polishinguniformity. If different areas on the substrate are polished atdifferent rates, then it is possible for some areas of the substrate tohave too much material removed (“overpolishing”) or too little materialremoved (“underpolishing”). In addition to planarization, polishing padscan be used for finishing operations such as buffing.

Polishing pads for CMP may include “standard” pads and fixed-abrasivepads. A standard pad may have as a polyurethane polishing layer with adurable roughened surface, and can also include a compressible backinglayer. In contrast, a fixed-abrasive pad has abrasive particles held ina containment media, and can be supported on a generally incompressiblebacking layer.

Polishing pads are typically made by molding, casting, or sinteringpolyurethane materials. In the case of molding, the polishing pads canbe made one at a time, e.g., by injection molding. In the case ofcasting, the liquid precursor is cast and cured into a cake, which issubsequently sliced into individual pad pieces. These pad pieces canthen be machined to a final thickness. Grooves can be machined into thepolishing surface, or be formed as part of the injection moldingprocess. More recently, three-dimensional printing techniques have beenproposed for manufacturing of polishing pads. The pads may be formedfrom acrylate polymers, as described in U.S. Pat. Nos. 10,399,201 and10,384,330, and US Patent Application Publication Nos. 2020/0001433 and2019/0047112.

SUMMARY

In implementations described herein, an interpenetrating network is usedto form a polishing pad. The properties of the polishing pad may beaccurately controlled by adjusting a ratio of cationic materials tofree-radical materials in a blend that is photopolymerized to form thepolishing pad. Further, different blends may be used in differentnozzles of a three-dimensional printer to adjust the properties indifferent regions of the polishing pad, such as the polishing surface,the body, or the backing, among others.

An implementation described herein provides a method of generating aformulation for a polishing pad including an interpenetrating polymernetwork (IPN). The method includes selecting a cationically polymerizedmaterial and a free-radically polymerized material to control propertiesof the polishing pad and blending the cationically polymerized materialwith the free-radically polymerized material to form a precursor blend,wherein a ratio of the cationically polymerized material to thepre-radically polymerized material in the precursor blend is selected tocontrol the properties of the polishing pad. A free radicalphotoinitiator and a cationic photoinitiator are blended with theprecursor blend to form an active blend, and the active blend isprovided to a manufacturer to generate the polishing pad using athree-dimensional printer.

Another implementation described herein provides a method ofmanufacturing a polishing pad including an interpenetrating polymernetwork (IPN). The method includes obtaining an active blend including afree radical photoinitiator, a free-radically polymerized material, acationic photoinitiator, a cationically polymerized material, wherein aratio of the free-radically polymerized material to the cationicallypolymerized material in the active blend is selected to controlproperties of the polishing pad. A raw polishing pad is formed from theactive blend, and the raw polishing pad is irradiated to initiate afree-radical photopolymerization of the free-radically polymerizedmaterial and a cationic photopolymerization of the cationicallypolymerized material.

Another implementation disclosed herein provides a polishing pad for asemiconductor fabrication operation. The polishing pad includes aninterpenetrating polymer network formed from a free radicallypolymerized material and a cationically polymerized material.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic cross-sectional side view of an example polishingpad.

FIG. 1B is a schematic cross-sectional side view of another examplepolishing pad.

FIG. 1C is a schematic cross-sectional side view of yet another examplepolishing pad.

FIG. 2 is a schematic side view, partially cross-sectional, of achemical mechanical polishing station.

FIG. 3 is a schematic side view illustrating a substrate in contact withthe polishing pad of FIG. 1A.

FIG. 4 is a drawing of an interpenetrating polymer network.

FIG. 5 is a block flow diagram of a method of forming a polishing pad bythe 3D printing of an interpenetrating polymer network.

FIG. 6 is a block flow diagram of a method for forming a polishing padwith an inherent window region by the 3D printing of an interpenetratingpolymer network.

FIG. 7A is a plot of Tan δ measured by dynamic mechanical analysis (DMA)of pure cationic polymers and interpenetrating polymer networks as afunction of increasing cationic polymer composition.

FIG. 7B is a plot of modulus values for E30 and E90 as a function of Tgand cationic polymer composition.

FIG. 8A is a plot of percent cure at the bottom of a droplet after UVcuring as a function of polymer composition.

FIG. 8B is a plot of percent cure at the bottom of a droplet after UVcuring and annealing as a function of polymer composition.

FIG. 9A is a plot of percent cure at the top of a droplet after UVcuring as a function of polymer composition.

FIG. 9B is a plot of percent cure at the top of the droplet after UVcuring and annealing as a function of polymer composition.

FIG. 10 is a plot of light transmittance as a function of wavelength(nm).

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

To polish a wafer, a polishing pad and polishing head apply mechanicalenergy to the substrate. The pad also helps to control the transport ofa slurry which interacts with the substrate during the polishingprocess. Because polishing pads are typically made from viscoelasticpolymeric materials, the mechanical properties of a polishing pad (e.g.,elasticity, rebound, hardness, and stiffness), and the chipmanufacturing process (CMP) processing conditions have a significantimpact on the CMP polishing performance on both an integrated circuit(IC) die level (microscopic or nanoscopic) and wafer or global level(macroscopic). For example, CMP process forces and conditions, such aspad compression, pad rebound, friction, and changes in temperatureduring processing, and abrasive aqueous slurry chemistries will impactpolishing pad properties and thus CMP performance.

Current pad materials and methods to produce them limit the manipulationand fine control bulk pad properties such as storage modulus (E′) andloss modulus (E″), which play critical roles in pad performance.Therefore, uniform CMP requires a pad material and polishing surfacewith a predictable and finely controlled balance of storage modulus E′and loss modulus E″, that are further maintained over a CMP processingtemperature range, from, for example, about 30° C. to about 90° C. Thepolishing surface also requires features, such as grooves and channels,to carry polishing liquid.

However, conventional pad production via traditional bulk polymerizationand casting and molding techniques only provide a degree of pad property(e.g., modulus) control, because the pad is a random mixture of phaseseparated macromolecular domains that are subject to intramolecularrepulsive and attractive forces and variable polymer chain entanglement.For example, the presence of phase separated macroscopic structuraldomains in the bulk pad may yield an additive combination of non-linearmaterial responses, such as a hysteresis in the storage modulus (E′)over multiple heating and cooling cycles that typically occur during theCMP processing of batches of substrates, which may result polishingnon-uniformities and unpredictable performance across the batch ofsubstrates.

Because of the drawbacks associated with conventional polishing pads andtheir methods of manufacture, there is a need for new polishing padmaterials and new methods of manufacturing polishing pads that providecontrol of pad feature geometry, and fine control of the pad's material,chemical and physical properties. Such improvements are expected toyield improved polishing uniformity at both a microscopic level andmacroscopic level, such as over the entire substrate.

Some aspects of the present disclosure are directed to a formulationcomprising monomers that form an interpenetrating polymer network (IPN)for the formation of a polishing layer of a polishing pad. In oneimplementation, the formulation is applied to form the polishing pad byan additive manufacturing process, such as three dimensional (3D)printing. In 3D printing, a printhead ejects droplets of the formulationonto a surface from a nozzle, then cures the droplets with light, e.g.,ultraviolet light, from a light source, such as an LED or focused lampin the printer. Although the 3D printing provides significantadvantages, in some implementations, the IPN formulation may be cast ina mold, then exposed to a bulk UV light for curing.

The technique may include preparing a blend that includes free radicallypolymerized materials, such as urethane acrylate oligomers, esteracrylates, or ether acrylates, among others. The blend also includescationically polymerized materials, such as epoxy oligomers. Othercomponents, including monomers, stabilizers, and the like, are thenadded to adjust properties of the blend, such as the viscosity. Freeradical and cationic photoinitiators are added to form an active blend.With the oligomers of the present techniques, the active blend may havea relatively low viscosity to facilitate 3D printing of the formulationfor manufacturing CMP polishing pads. The formulation may then be 3Dprinted to make the CMP Pads. The CMP pads may have a high modulus andgood elasticity for semiconductor applications.

Referring to FIG. 1A-1C, a polishing pad 102 includes a polishing layer104. As shown in FIG. 1A the polishing pad can be a single-layer padthat consists of the polishing layer 104, or as shown in FIG. 1C thepolishing pad can be a multi-layer pad that includes the polishing layer104 and at least one backing layer 106.

The polishing layer 104 can be a material that is inert in thepolishing. The material of the polishing layer 104 can be a plastic,e.g., an interpenetrating network formed from a blend of a urethaneacrylate oligomer and in epoxy oligomer. In some implementations thepolishing layer 104 is a relative durable and hard material. Forexample, the polishing layer 104 can have a hardness of about 40 to 80,e.g., 50 to 65, on the Shore D scale.

As shown in FIG. 1A, the polishing layer 104 can be a layer ofhomogeneous composition, e.g., formed from the IPN with no additionalcomponents. In some implementations, the polishing layer includes pores,e.g., small voids. The pores can be 50-100 microns wide. Such pores neednot detract from the generally homogeneous composition of the polishinglayer 140.

As shown in FIG. 1B the polishing layer 104 can include abrasiveparticles 108 held in a matrix 110 of plastic material, e.g., aninterpenetrating network of polyurethane and epoxide. The abrasiveparticles 108 are harder than the material of the matrix 110. Theabrasive particles 108 can be from 0.05 weight percent (wt. %) to 75 wt.% of the polishing layer. For example, the abrasive particles 108 can beless than 1 wt. % of the polishing layer 104, e.g., less than 0.1 wt. %.Alternatively, the abrasive particles 108 can be greater than 10 wt. %of the polishing layer 104, e.g., greater than 50 wt. %. The material ofthe abrasive particles can be a metal oxide, such as ceria, alumina, orsilica, or any combinations thereof.

The polishing layer 104 can have a thickness D1 of 80 mils or less, 50mils or less, or 25 mils or less. Because the conditioning process tendsto wear away the cover layer, the thickness of the polishing layer 104can be selected to provide the polishing pad 102 with a useful lifetime,e.g., 3000 polishing and conditioning cycles.

On a microscopic scale, the polishing surface 112 of the polishing layer104 can have rough surface texture, e.g., a root mean squared (rms)surface-roughness of 2-4 microns. For instance, the polishing layer 104can be subject to a grinding or conditioning process to generate therough surface texture. In addition, 3D printing can provide smalluniform features, e.g., down to 200 microns.

Although the polishing surface 112 can be rough on a microscopic scale,the polishing layer 104 can have good thickness uniformity on themacroscopic scale of the polishing pad itself. This uniformity may referto the global variation in height of the polishing surface 112 relativeto the bottom surface of the polishing layer, and does not count anymacroscopic grooves or perforations deliberately formed in the polishinglayer. The thickness non-uniformity can be less than 1 mil.

In some implementations, at least a portion of the polishing surface 112can include a plurality of grooves 114 formed therein for carryingslurry. The grooves 114 may be of nearly any pattern, such as concentriccircles, straight lines, a cross-hatched, spirals, and the like. Inimplementations with grooves present, then on the polishing surface 112,the plateaus between the grooves 114 can be, for example, 25-90% of thetotal horizontal surface area of the polishing pad 102. Thus, thegrooves 114 can occupy 10%-75% of the total horizontal surface area ofthe polishing pad 102. The plateaus between the grooves 114 can have alateral width of about 0.1 to 2.5 mm.

In some implementations, e.g., if there is a backing layer 106, thegrooves 114 can extend entirely through the polishing layer 104. In someimplementations, the grooves 114 can extend through about 20-80%, e.g.,at 40-60%, of the thickness of the polishing layer 104. The depth of thegrooves 114 can be 0.25 to 1 mm. For example, in a polishing pad 102having a polishing layer 104 that is 40-60 mils thick, e.g., 50 milsthick, the grooves 114 can have a depth D2 of about 15-25 mils, e.g., 20mils.

The backing layer 106 can be softer and more compressible than thepolishing layer 104. The backing layer 106 can have a hardness of 80 orless on the Shore A scale, e.g., a hardness of about 60 Shore A or less.The backing layer 106 can be thicker or thinner than (or the samethickness as) the polishing layer 104.

In certain implementations, the backing layer 106 can be an open-cell ora closed-cell foam, such as polyurethane or polysilicone with voids, sothat under pressure the cells collapse and the backing layer compresses.Examples of material for the backing layer are PORON 4701-30 from RogersCorporation, in Rogers, Conn., or SUBA-IV from Rohm & Haas. The hardnessof the backing layer 106 can generally be adjusted by selection of thelayer material and porosity. Alternatively, the backing layer 106 can beformed from the same precursor and have the same porosity as thepolishing layer, but have a different degree of curing, or a differentIPN composition, so as to have a different hardness. In theseimplementations, the ratio between the free radically polymerizedoligomer and the cationically polymerized oligomer can be adjusted tochange the modulus of the backing material.

Turning now to FIG. 2 , one or more substrates 202 can be polished at apolishing station 204 of a CMP apparatus. A description of an applicablepolishing apparatus can be found in U.S. Pat. No. 5,738,574.

The polishing station 204 can include a rotatable platen 206 on which isplaced the polishing pad 102. During polishing, a polishing liquid 208,e.g., abrasive slurry, can be supplied to the surface of polishing pad102 by a slurry supply port or combined slurry/rinse arm 210. Thepolishing liquid 208 can contain abrasive particles, a pH adjuster, orchemically active components.

The substrate 202 is held against the polishing pad 102 by a carrierhead 212. The carrier head 212 is suspended from a support structure,such as a carousel, and is connected by a carrier drive shaft 214 to acarrier head rotation motor so that the carrier head can rotate about anaxis 216. As the carrier head 212 is rotated about the axis 216, therotatable platen 206 is rotated by a platen axis 218 around a verticalaxis 220. The relative motion of the polishing pad 102 and the substrate202 in the presence of the polishing liquid 208 results in polishing ofthe substrate 202.

An in-situ monitoring system, e.g., an optical endpoint monitoringsystem, eddy current monitoring system, motor current monitoring system,etc., can be used in a CMP process to determine when a substrate 202 ora bulk film has been polished to a desired thickness when the contactmaterial has been removed from the field (upper surface) of a layer.This can be performed using optical techniques, for example, in which alight beam 222 is directed towards the substrate 202 through a window224 in the rotatable platen 206 and through a polishing pad window 226.The light beam 222 may be intermittently reflected from the surface ofthe substrate 202 as the rotatable platen 206 rotates. An optical system228 may use interferometry or changes in the reflectance of thesubstrate 202 to determine the removal of material from the surface ofthe substrate 202. As described herein, the polishing pad window 226 isgenerally made from the same material as the polishing layer 104, forexample, formed by 3D printing of the polishing pad window 226 duringthe 3D printing of the polishing pad 102. For optical clarity, thepolishing pad window 226 may include a lower amount of photoinitiators,which absorb light. Further, the polishing pad window 226 may notinclude the abrasive particles 108, described with respect to FIG. 1 .

Referring to FIG. 3 , at least the polishing layer 104 (FIG. 1 ) of thepolishing pad 102 is manufactured utilizing 3D printing. In themanufacturing, thin layers of material are progressively deposited andfused. For example, droplets 302 of a formulation of pad precursormaterial can be ejected from a nozzle 304 of a droplet ejecting printer306 to form a layer 308. The droplet ejecting printer 306 is similar toan inkjet printer, but employs the pad precursor material rather thanink. The nozzle 304 translates (shown by arrow A) across a support 310.

For a first layer 312 deposited, the nozzle 304 can eject onto thesupport 310. For subsequently deposited layers 314, the nozzle 304 caneject onto the already solidified material 316. After each layer 308 issolidified, a new layer is then deposited over the previously depositedlayer until the full 3-dimensional polishing layer 104 is fabricated.Each layer is applied by the nozzle 304 in a pattern stored in a 3Ddrawing computer program that runs on a computer 318. Each layer 308 isless than 50% of the total thickness of the polishing layer 104, e.g.,less than 10%, e.g., less than 5%, e.g., less than 1%.

During the deposition of the layers 312 and 314, a window region 320 maybe deposited. The window region 320 may be formed from an IPN blend witha lower concentration of photoinitiators to improve the transparency forthe measurement wavelengths, e.g., with a cutoff below 350 nm, below 375nm, or below 400 nm. As used herein, a cutoff indicates a point whereinthe transmittance drops below 50% of the transmittance at 800 nm as thewavelength is decreased. Further, the material used in the window regionmay not include abrasive particles, further increasing the lighttransmittance. The window region 320 can be located in the groove(s)between raised plateaus as illustrated, or the raised plateaus canprovide part of the window region 320.

To change the material deposited on the pad, the droplet ejectingprinter 306 may include additional nozzles that are coupled toreservoirs holding different materials. For example, one reservoir mayhold a normal IPN blend material used to form most of the polishinglayer 104, while a second reservoir feeding another nozzle may includean IPN blend material with a lower concentration of the photoinitiators,and no abrasives. For further customization of the properties of thepads, additional nozzles may print only the epoxy oligomer blend with acationic photoinitiator, while other nozzles may print only the urethaneacrylic oligomer with the free radical photoinitiator, allowingcustomization of the blend used for a region. Other combinations, suchas changes in a blend ratio that includes both epoxy oligomers andurethane acrylic oligomers, may be used.

The support 310 can be a rigid base, or be a flexible film, e.g., alayer of polytetrafluoroethylene (PTFE). If the support 310 is a film,the support 310 can form a portion of the polishing pad 102. Forexample, the support 310 can be the backing layer 106 or a layer betweenthe backing layer 106 and the polishing layer 104. Alternatively, thepolishing layer 104 can be removed from the support 310, and applied toa backing layer 106.

Solidification can be accomplished by polymerization. For example, thelayer 308 of pad precursor material can be a blend that includes anepoxy oligomer and a urethane oligomer, along with the relevantphotoinitiators. The blend can be polymerized in-situ by light, e.g.,ultraviolet (UV) curing using a UV source 322. The UV source 322 mayemit a broad UV spectrum, including, for example, UV-A and UV-B, orUV-A, UV-B, and UV-C. In some implementations, a narrow bandwidthsource, such as a UV LED laser may be used.

The pad precursor material can be cured effectively immediately upondeposition, or an entire layer 308 of pad precursor material can bedeposited and then the entire layer 308 be cured simultaneously.Further, a partial cure may be effected during printing, with a fullcure being carried out by annealing under a large UV source, or in anoven. As the epoxy oligomers cure more slowly that the urethane acrylateoligomers, a hold time after printing, even without additional light orheat annealing, may be used to achieve full properties.

In addition to 3D printing, other technologies may be used to form thepolishing layer 104 from the IPN blend, as described herein. Forexample, the polishing layer 104 may be cast by placing the IPN blend ina mold, and irradiating to form the final shape. A window may be formedusing leaving a circular mold to form a hole in the polishing layer 104during the casting procedure. After the polishing layer 104 is cured,the hole can be filled with an IPN blend that includes a lower amount ofphotoinitiator. Curing the window material may couple it to the polymerstructure of the surrounding polishing layer 104.

The 3D printing generally avoids the need for making molds, which can berelatively expensive and add time in the manufacturing. The 3D printingmay eliminate several conventional-pad manufacturing steps, such asmolding, casting, and machining. Further, in conventional methods,windows may pop out during CMP or may not be coplanar with respect tothe surface of the pads resulting in uneven surfaces. Additionally,tight tolerances can generally be achieved in 3D printing due to thelayer-by-layer printing. Also, one printing system (with printer 306 andcomputer 318) can be employed to manufacture a variety of differentpolishing pads, simply by changing the pattern stored in the 3D drawingcomputer program in certain implementations. Further, the use ofadditional nozzles can allow for customization of properties during theprinting process.

In some implementations, the backing layer 106 can also be fabricated by3D printing. For example, the backing layer 106 and polishing layer 104could be fabricated in an uninterrupted operation by the printer 306.The backing layer 106 can be provided with a different hardness than thepolishing layer 104, for example, by applying a different amount ofcuring, e.g., a different intensity of UV radiation. In someimplementations, the backing layer is formed using a different IPNblend. The backing layer 106 may also include a window region, alignedwith the window region 320 in the polishing layer 104.

In other implementations, the backing layer 106 is fabricated by aconventional process and then secured to the polishing layer 104. Forinstance, the polishing layer 104 can be secured to the backing layer106 by a thin adhesive layer, e.g., as a pressure-sensitive adhesive.

FIG. 4 is a schematic drawing of an interpenetrating polymer network(IPN) 400 of two cross-linked polymers. As used herein, an IPN 400 maybe defined as an interpenetrating network of two or more polymers, suchas polymers 402 and 404, in which each of the polymers 402 and 404 iscrosslinked with other polymer chains of substantially the same type,i.e., polymer chains of polymer 402 are cross-linked with other polymerchains of polymer 402, and polymer chains of polymer 404 arecross-linked with other polymer chains of polymer 404. Some small amountof cross-linkage between polymers 402 and 404 is permissible, e.g., lessthan 5%, e.g., less than 1% of the linkages. An IPN 400 may be formedfrom a blend of monomers or oligomers, e.g., the IPN blend, as usedherein, with at least one of the polymers synthesized in the presence ofanother.

The IPN 400 may be formed using cross-linking polymerization of twomultifunctional oligomers that polymerize through different mechanisms,such as free radical polymerization and cationic polymerization. In theexample of FIG. 4 , a polyurethane acrylate oligomer is cross-linkedthough free-radical links 406 to form polymer 402, while an epoxyoligomer is cross-linked through cationic links 408 to form aninterpenetrating polymer 404. This produces a “physically cross-linked”network wherein cross-linked polymer chains of one polymer, e.g.,polymer 402, are entangled with and/or penetrate the network formed bycross-linked chains of the other polymer, e.g., 404. Each individualnetwork retains its individual properties, so that synergisticimprovements in properties including E′30, E′90, E′30/E′90, strength,toughness, compression and elongation may be made through changes to theconcentrations of the components.

An IPN 400 can be distinguished from the other multiple systems ornetworks through their multi-continuous structure ideally formed by thephysical entanglement or interlacement of at least two polymers that arein intimate physical contact but may or may not be chemically bounded toone another. Photoinitiated polymerization has been a very efficient wayto obtain IPNs and is initiated by a monomer mixture such as acrylates,which polymerize by a radical mechanism, and epoxides, which polymerizeby a cationic mechanism. UV irradiation has been explored heavily toproduce IPNs.

FIG. 5 is a method 500 of forming a polishing pad from an IPN. Thepolishing pad may be a CMP pad. The method 500 begins at block 502, withthe selection of a urethane acrylate oligomer and an epoxy oligomer. Theoligomers are chosen to control the properties of a polishing pad.

Typical material composition properties that can be selected using themethods and material compositions described herein include storagemodulus E′, loss modulus E″, hardness, tan δ, yield strength, ultimatetensile strength, elongation, glass transition temperature (Tg) andother related properties. It is desirable for the window material tohave a similar storage modulus as the surrounding polishing elements sothat the window material wears at a similar rate and does not extendabove or below the surface or the polishing pad over the lifetime. Theproperties that can be controlled by include the modulus, or stiffness,of the polishing pad. Different moduli can be selected for differentapplications and lifespan. Generally, three modulus ranges may be used,as shown in Table 1.

TABLE 1 Moduli used for polishing pads Low Modulus Medium Modulus HighModulus Compositions Compositions Compositions E′30 5 MPa-100 MPa 100MPa-500 MPa 500 MPa-3000 MPa

The moduli may be controlled by the selection of the urethane acrylateoligomer and the epoxy oligomer as well as by the ratio of the urethaneacrylate oligomer to the epoxy oligomer. Other properties may becontrolled by the selections of the oligomers as well as the ratios ofthe oligomers used to form the IPNs. Accordingly, the control of ratiosand oligomers allow the properties of polishing pads to be tuned tocreate a desired composite of properties within a layer and/or layer bylayer, such as those properties including E′30, E′90, E′30/E′90,strength, toughness, compression, and elongation.

In various implementations, the IPN blend, or active blend, includes amaterial that is polymerized by a free radical mechanism, such as aurethane acrylate oligomer, an ester acrylate, an ether acrylate, ormonomer. The IPN blend also includes a material that is polymerized by acationic mechanism, such as an epoxy oligomer. The IPN blend alsoincludes a free radical photoinitiator and a cationic photoinitiator.Each of the oligomers, such as the urethane acrylate oligomer and theepoxy oligomer, can be part of a blend of components. In someimplementations, the IPN blend also includes a monomer and an additive.The additive may be, for example, silicones, polyether silicones, orsurfactants. In implementations, the formulation need not include asolvent as solvent can act as plasticizer to reduce or adversely affectmechanical properties.

In various implementations described herein, the cationicallypolymerized formulation includes 3,4-epoxycyclohexylcarboxylate(available as Celloxide 2021P from Daicel Corp of Tokyo),3-ethyl-3-hydroxymethyl-oxetane (available as OXT 101 from Toagosei Co.of Tokyo) and polycaprolactone (available in the PLACCEL line ofproducts from Daicel). Other monomers or oligomers may be used inaddition to or in place of these, including, for example, Celloxide2080, Celloxide 2000, OXT 101, OXT 121, OXT 221, Placcel 205U, Placcel305, or polypropylene glycol, among others.

In various implementations described herein, the free radicallypolymerized formulation includes a polyester urethane acrylate, such asBR744SD, BR744BT, BRC 841, BRC 843, BR 741, BR742S, BR 771F, or BR7432G130, among others available in the Bomar product line from DymaxOligomers and Coatings of Torrington, Conn., USA. Other materials thatmay be used in the free radically polymerized formulation includetrimethylolpropane triacrylate (TMPTA) (available as SR351H fromSartomer Arkema of Exton, Pa., USA), and CN 8881, 9004, 9009, 99030,9031, 964, 981, 991, or 996 (available in the Sartomer N3XT dimensionseries from Sartomer). Any number of other monomers and oligomers, suchas acrylic acid, may be included in the formulation. Further,crosslinking agents used in the formulations could include EB40 fromAllnex, TPGDA, or multifunctional acrylates, among others.

In various implementations described herein, the free-radical, oracrylate, photoinitiators used include an acyl phosphine oxide (such asOmnirad 819 available from IGM Resins of Waalwijk, The Netherlands), analpha hydroxy ketone (such as Omnirad 2959, available from IGM Resins),or 2-Methyl-4′-(methylthio)-2-morpholinopropiophenone (available asOmnirad 907 from IGM Resins). Similar free radical photoinitiators maybe used in addition to, or in combination with, these initiators. Thechoice of the photoinitiator may be made based on the viscosity of theresulting solution and the absorbance of the photoinitiator, such as ina window application as described herein.

In various implementations described herein, the cationic photoinitiatoris Omnicat 250, available from IGM Resins. Omnicat 250 is a 75% blend of(4-methylphenyl)[4-(2-methylpropyl) phenyl]-, hexafluorophosphate(1-) in25% propylene carbonate. Other cationic photoinitiators may be usedinstead of, or in addition to, Omnicat 250, such as other cationicphotoinitiators in the Omnicat product line from IGM Resins, including,for example, sulfonium hexafluoro phosphate (Omnicat 270), or Omnicat250, 440, 432, BL 550, and 320, among others. The choice of thephotoinitiator may be made based on the viscosity of the resultingsolution, and the absorbance of the photoinitiator, such as in a windowapplication as described herein.

At block 504, the IPN blend is made from the urethane acrylate oligomerand the epoxy oligomer at a ratio to control the properties of thepolishing pad. For example, increasing the amount of the epoxy oligomerin the IPN blend may increase the stiffness of the resulting polishingpad. Further, the IPN blend may be a viscous liquid, in which theviscosity can be adjusted by modifying the ratio of the urethaneacrylate oligomer to the epoxy oligomer. The choice of the urethaneacrylate oligomer and the epoxy oligomer may also be used to adjust theviscosity of the IPN blend. For example, the IPN blend used for 3Dprinting may be a free-flowing viscous liquid at a viscosity in a rangeof about 10 centipoise (cP) to a maximum value of about 20 cP at atemperature of 70° C. The polishing layer fabricated by the 3D printingutilizing the formulation may have an elongation at break of at least 8%and ultimate tensile strength (UTS) of at least about 30 megapascals(MPa). The correlation of properties to the ratio of the oligomers inthe IPN plant is discussed further with respect to the examples.

At block 506, the IPN blend is mixed with the photoinitiators to form anactive blend. As described herein the photoinitiators include freeradical initiators, for example, used to initiate the polymerization ofthe urethane acrylate oligomer. The photoinitiators also includecationic photoinitiators, for example, used to initiate thepolymerization of the epoxy oligomers. Further, as described withrespect to FIG. 6 , a different IPN blend may be used for differentportions of the polishing pad, such as a window used for endpointdetection in the polishing process.

At block 508, the polishing pad is made, for example, by a manufacturerat a semiconductor fabrication plant. This may be a single step, forexample, during a 3D printing process using the printing of droplets ofthe active blend to form the polishing pad. In some implementations, apolishing pad may be cast from an IPN blend. This polishing pad isformed as described at block 510. At block 512 the polishing pad isirradiated to initiate photopolymerization, e.g., curing. At block 510,droplets may be ejected from the nozzles at target locations asdetermined by a file in the computer that includes 3D plans for thepolishing pad.

At block 512, the formulation having the urethane acrylate oligomer andepoxy oligomer is subjected to photopolymerization to form the layer ofthe polishing pad. The irradiation may be performed at different levelsduring the process. For example, an initial radiation may be performedto fix droplets in place, while a more intense radiation may be used tocure the entire layer, either after the layer is deposited, or after theentire structure is created.

Once the 3D printing of the polishing pad is completed, the polishingpad may be annealed under a UV lamp or in an oven for further curing. Itcan be noted that the curing will continue to take place after theformation of the polishing pad, even without annealing. Accordingly, thepolishing pad may be allowed to rest before use to allow for furthercure time. As described herein, the additives and oligomer ratio may beused to adjust the viscosity and curability of the formulation and themechanical properties of the polishing pad.

FIG. 6 is a block flow diagram of a method 600 for forming a polishingpad with an inherent window region by the 3D printing of aninterpenetrating polymer network. The method begins at block 602 withthe formation of the polishing layer IPN blend. This may be done asdescribed herein, for example, using the ingredients discussed withrespect to FIG. 5 . The proportions of the ingredients used may be asdescribed with respect to the examples.

At block 604, the window IPN blend is formed. Generally, the proportionsof the oligomers and other ingredients in the window IPN blend willmatch the polishing layer IPN blend, so that the properties will alsomatch. However, the amounts of photoinitiators in the window IPN blendmay be lower than in the polishing layer IPN blend, as thephotoinitiators may absorb light in a region used for the endpointdetection for the polishing operation. For example, in variousimplementations, the photoinitiators may be decreased by about 5% withrespect to the polishing layer IPN, or by about 10%, or by about 20%, ormore. This will slow down the curing of the window region with respectto the rest of the polishing layer. The blends are then loaded into the3D printer, for example, in reservoirs for at least two differentdroplet ejectors in the nozzle. Once the blends are loaded, thepolishing pad with the inherent window region is printed.

At block 606, the location of the nozzle is determined and compared to astored 3D plan of the polishing pad. At block 608, a determination ismade as to whether the nozzle is over a window region of the polishingpad. If so, at block 610, a droplet of the window IPN blend is depositedat the target location. If not, at block 612, a droplet of the polishinglayer IPN blend is deposited with target location.

Once the droplet is deposited, at block 614, the target location isirradiated to initiate photopolymerization and fix the droplet in place.This may be done using a number of different techniques. In animplementation, a lower power irradiation may start a top cure of thephotopolymerization, which may be used just to fix the droplet in placewhile other droplets are deposited. This may be then followed by a moreintense irradiation to initiate a full cure, or bottom cure, once theentire polishing pad is formed. In another implementation, a curing lampmay be passed over the polishing pad once a full layer has been printedto start a top cure of that layer. It can be noted that upper layersprinted on lower layers that have already started to cure may bond tothe lower layers shortly after being printed, through the reactions thathave already initiated in the lower layers.

Examples

The examples are given only as examples and not meant to limit thepresent techniques. A number of different mixtures were made todetermine the properties of the IPNs as the ratios of the componentswere varied. The components used were as discussed with respect to FIG.5 .

The experiments were carried out by mixing compositions containing theradical and cationic components in the proportions shown in Table 1. Theformulations were cured bulk in a silicone mold of Type V dogbones (withthickness of 2 mm) by exposing to UV light of about 1150 mJ/cm² in aHeraeus UV curing station with 3 passes at 18 ft/min (each pass exposedat 380 mJ/cm2). In practice, these formulations may be 3D printed andthen cured to make CMP Pads, including CMP Pads having a high modulusand good elasticity for semiconductor applications. Accordingly the UVdosage can be reduced depending up on the thickness of samples.

Dynamic mechanical analysis (DMA) was performed on the UV-cured films.The main parameters that can be obtained are E and tan δ. E′ is thestorage modulus, the elastic component of the complex modulus, E. Tan δis the damping factor, calculated as the ratio E″/E′, in which E″ is theloss modulus, the dissipative component of the complex modulus E′ of theviscoelastic material. Importantly, the molecular weight, chain lengthand branching of the polymer may play a role in the weight percent ofpolymer due to such factors that include polymer miscibility and mixtureviscosity.

TABLE 1 Compositions used to test properties of IPNs for polishing pads.Radical composition Cationic composition Radical Cationic BR CelloxideOXT composition composition 744SD SR 351H 2021 P 101 PCL-205U (wt. %)(wt. %) (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) 100 0 20 80 0 0 0 90 1018 72 5.7 1.3 3 75 25 15 60 14.25 3.25 7.5 55 45 11 44 25.65 5.85 13.535 65 7 28 37.05 8.45 19.5 0 100 0 0 57 13 30 Note that 0.5 wt. % ofeach of the cationic and radical photoinitiators were used.

As described herein, IPNs are used to tune and adjust the properties ofpolishing pads to create a desired composite of properties within alayer and/or layer by layer, such as those properties including E′30,E′90, E′30/E′90, strength, toughness, compression, and elongation.

FIG. 7A is a plot of Tan δ as measured by dynamic mechanical analysis(DMA) of pure cationic polymers and interpenetrating polymer networks asa function of increasing cationic polymer composition. As shown in FIG.7A, the 100% cationic, or pristine cationic polymer 702, has the highestTg at 115° C. However, a 10% cationic polymer 704 is nearly as high, ata Tg of 100° C. Increasing to a 25% cationic polymer 706 drops the Tgfurther, to a value of 78° C. A 45% cationic polymer 708 has a Tg ofabout 62° C., and a 65% cationic polymer has a Tg of about −65° C. Thus,as the cationic polymer content increases (arrow 712) from 10% to 65%,the Tg falls. It can be noted that the kinetics of polymerization arefaster for the free-radical materials, inhibiting the formal of theepoxy network. Accordingly, the properties are not linearly related tothe concentration ratios of the free-radically polymerized materials tothe cationically polymerized materials.

In the glass-transition-temperature (Tg) region a strong decrease of E′can be observed as the cationic polymer content increases (arrow 712),while tan δ shows a maximum. The temperature corresponding to such amaximum was assumed to be the Tg of the cured film. In FIG. 7A, the tanδ curves for the pure-epoxy and the IPN system are shown. In the case ofthe hybrid system, a broad Tg peak can be observed, with intervals inbetween the Tg interval of the pure acrylic and the pure epoxy UV-curedresins. This indicates a complicated polymer-network structure. Asdescribed herein, the properties of the IPN networks are not an averageof the properties of the two component networks. The IPN networks have acomplicated polymer network structure owing to the differences inkinetics of polymerization as a function of individual compositions.Faster polymerization of one component limits mass transfer and movementof other fragments through the vitrified matrix.

FIG. 7B is a plot of modulus values for E30 and E90 as a function of Tgand cationic polymer composition. The selection, formulation and/orformation of materials that have a desirable storage modulus E′ andE′30:E′90 ratio in desirable regions of an advanced polishing pad by useof an additive manufacturing process is an important factor in assuringthat the polishing results achieved by the advanced polishing pad areuniform across a substrate. It is noted that storage modulus E′ is anintrinsic material property of a formed material, which results from thechemical bonding within a cured polymeric material. Storage modulus maybe measured at a desired temperature, such as 30° C. and 90° C. using adynamic mechanical analysis (DMA) technique. Examples of formulationsthat contain different storage moduli are illustrated in FIG. 1B).

As shown in the plots of FIGS. 7A and 7B, increasing the cationic ratioaffords IPNs with lower Tg. This is dictated by the kinetics of acrylatevs epoxy polymerization during the UV curing process. This is furtherexplored in the measurements shown in FIGS. 8A, 8B, 9A, and 9B.

FIG. 8A is a plot of percent cure at the bottom of a droplet after UVcuring as a function of polymer composition. FIG. 8B is a plot ofpercent cure at the bottom of a droplet after UV curing and annealing asa function of polymer composition. FIG. 9A is a plot of percent cure atthe top of a droplet after UV curing as a function of polymercomposition. FIG. 9B is a plot of percent cure at the top of the dropletafter UV curing and annealing as a function of polymer composition.

As shown in these figures, the top cures for cationic polymers arebetter while through cures are better for radical curing. Acrylates curefaster than epoxy and inhibit the epoxy network formation by limitingdiffusion (glassy), resulting in materials with the lowest Tg. The UVcured samples were annealed for two hours and this was found to increasetop surface cures due to reduced moisture and oxygen inhibition.Further, the formulations were tuned to have viscosities between 13-15cP which makes them amenable to inkjet printing.

Window Formulation Tests

In a similar fashion to the more general polishing pad IPN formulations,the window region formulations were tested by mixing compositionscontaining the free radical and cationic components, and curing films ofabout 2 mm in thickness by exposing them to UV light of 1150 mJ/cm². Themajor difference was the amount of photoinitiator used in each test. Forexample, results for a window formulation using 0.1 weight percentOmnirad 819 are shown in Table 2.

TABLE 2 mechanical properties of IPN formulations for pad window curedusing 0.1 weight percent OmniRad 819. Viscosity after stability Visc@70test@70 UTS E30 E90 % epoxy % acrylate (cP) (cP) (MPa) % EL (MPa) (MPa)conversion conversion 12.9 13.2 29.6 2.8 1147 700 T: 85 T: 89 B: 75 B:93

It can be further added that the tuning of the Tg can be used to obtainpad window compositions with varying mechanical properties. Theviscosities of the above formulations are in the optimum range forinkjet printing, for example, between about 12 cP and about 16 cP,depending on the printhead. The thermal stability of these formulationscan be improved by adding inhibitors, for example, Proton Sponge®,1,8-bis(dimethylamino)naphthalene from Sigma-Aldrich, or a hinderedamine light stabilizer (HALS), such as Omnistab LS292 from IGM Resins.

The jetting performance of these formulations were tested, and they werefound to stably jet at 70 degrees with a drop velocity of 5.87 m/s. Theprint performance and the mechanical properties were validated using aConnex 500 printer from Stratasys at a concentration of 0.2 wt. % of theOmnirad 819 photoinitiator, providing the mechanical propertiestabulated in Table 3.

TABLE 2 mechanical properties of IPN formulations for pad window printedon Connex 500 using 0.2 weight percent Omnicure 819. Viscosity afterstability UTS % EL UTS % EL Visc@70 test@70 (Type V) (Type V, 5 (TypeIV) (Type IV, 5 E30 E90 % epoxy % acrylate (cP) (cP) (MPa) mm/min) (MPa)mm/min) (MPa) (MPa) conversion conversion 13.4 13.6 48.9 6.4 44 6.2 1380530 T: 70 T: 65 B: 77 B: 69 Note that the dogbone test samples areprepared according to ASTM D638 (Type V and Type IV).

FIG. 10 is a plot of polymer transmittance as a function of wavelength(nm). Finally, the cutoff wavelength and transmittance data for the padwindow are shown in FIG. 10 . The cutoff wavelength was 375 nm with anoptical transmittance of about 50% at wavelength >400 nm while those forcommercial windows is 395 nm with a transmittance of 50% atwavelengths >400 nm. Further, the cutoff does not change after 6 millionflashes of UV, which is after 45 second exposure accounting for one padlife or even after 90 second exposure indicating the stability of thepad window.

An implementation described herein provides a method of generating aformulation for a polishing pad including an interpenetrating polymernetwork (IPN). The method includes selecting a cationically polymerizedmaterial and a free-radically polymerized material to control propertiesof the polishing pad and blending the cationically polymerized materialwith the free-radically polymerized material to form a precursor blend,wherein a ratio of the cationically polymerized material to thepre-radically polymerized material in the precursor blend is selected tocontrol the properties of the polishing pad. A free radicalphotoinitiator and a cationic photoinitiator are blended with theprecursor blend to form an active blend, and the active blend isprovided to a manufacturer to generate the polishing pad using athree-dimensional printer.

In an aspect, the properties include elongation at break, ultimatetensile strength (UTS), storage modulus, or glass transitiontemperature, or any combinations thereof. In an aspect, a maximum valueof a viscosity of the active blend is 20 centipoise (cP).

Another implementation described herein provides a method ofmanufacturing a polishing pad including an interpenetrating polymernetwork (IPN). The method includes obtaining an active blend including afree radical photoinitiator, a free-radically polymerized material, acationic photoinitiator, a cationically polymerized material, wherein aratio of the free-radically polymerized material to the cationicallypolymerized material in the active blend is selected to controlproperties of the polishing pad. A raw polishing pad is formed from theactive blend, and the raw polishing pad is irradiated to initiate afree-radical photopolymerization of the free-radically polymerizedmaterial and a cationic photopolymerization of the cationicallypolymerized material.

In an aspect, the free-radically polymerized material includes aurethane acrylate material. In an aspect, the cationically polymerizedmaterial includes an epoxy oligomer.

In an aspect, the method includes forming the raw polishing pad byprinting droplets of the active blend from a 3D printer nozzle to form alayer in a 3D printer and irradiating the raw polishing pad to initiatethe free-radical photopolymerization and the cationicphotopolymerization of the droplets.

In an aspect, the polishing pad is annealed to complete curing of theinterpenetrating polymer network. In an aspect, the polishing pad isannealed by irradiating the polishing pad. In an aspect, a viscosity ofthe active blend is less than 20 centipoise (cP).

Another implementation disclosed herein provides a polishing pad for asemiconductor fabrication operation. The polishing pad includes aninterpenetrating polymer network formed from a free radicallypolymerized material and a cationically polymerized material.

In an aspect, the free radically polymerized material includes aurethane acrylate oligomer. In an aspect, the cationically polymerizedmaterial includes an epoxy oligomer.

In an aspect, a ratio of the cationically polymerized material to thefree-radically polymerized material is controlled to achieve a targetproperty. In an aspect, the ratio of the cationically polymerizedmaterial to the free-radically polymerized material is between 10 wt. %and 65 wt. %. In an aspect, the ratio of the cationically polymerizedmaterial to the free radically polymerized material is about 25 wt. %.

In an aspect, the target property is an E′30 modulus. In an aspect, theE′30 modulus is between 5 megapascals (MPa) and 100 MPa. In an aspect,the E′30 modulus is between 101 megapascals (MPa) and 500 MPa. In anaspect, the E′30 modulus is between 501 megapascals (MPa) and 3000 MPa.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure. For example, either thepolishing pad, or the carrier head, or both can move to provide relativemotion between the polishing surface and the substrate. The polishingpad can be a circular or some other shape. An adhesive layer can beapplied to the bottom surface of the polishing pad to secure the pad tothe platen, and the adhesive layer can be covered by a removable linerbefore the polishing pad is placed on the platen. In addition, althoughterms of vertical positioning are used, it should be understood that thepolishing surface and substrate could be held upside down, in a verticalorientation, or in some other orientation. Accordingly, otherimplementations are within the scope of the following claims.

What is claimed is:
 1. A polishing pad for a semiconductor fabricationoperation, comprising an interpenetrating polymer network formed from afree radically polymerized material and a cationically polymerizedmaterial, wherein a ratio of the cationically polymerized material tothe free-radically polymerized material in the precursor blend isselected to control the properties of the polishing pad.
 2. Thepolishing pad of claim 1, wherein the properties comprise elongation atbreak, ultimate tensile strength (UTS), storage modulus, or glasstransition temperature, or any combination thereof.
 3. The polishing padof claim 1, wherein the free radically polymerized material comprises aurethane acrylate oligomer.
 4. The polishing pad of claim 1, wherein thecationically polymerized material comprises an epoxy oligomer.
 5. Thepolishing pad of claim 1, wherein the ratio of the cationicallypolymerized material to the free-radically polymerized material isbetween 10 wt. % and 65 wt. %.
 6. The polishing pad of claim 5, whereinthe ratio of the cationically polymerized material to the free radicallypolymerized material is about 25 wt. %.
 7. The polishing pad of claim 1,wherein the interpenetrating polymer network has an E′30 modulus between5 megapascals (MPa) and 100 MPa.
 8. The polishing pad of claim 1,wherein the interpenetrating polymer network has an E′30 modulus between101 megapascals (MPa) and 500 MPa.
 9. The polishing pad of claim 1,wherein the interpenetrating polymer network has an E′30 modulus between501 megapascals (MPa) and 3000 MPa.
 10. The polishing pad of claim 1,wherein the interpenetrating polymer network is formed from an activeblend comprising a free radical photoinitiator, a free-radicallypolymerized material, a cationic photoinitiator, and cationicallypolymerized material.
 11. The polishing pad of claim 10, wherein theinterpenetrating polymer network is formed by depositing droplets of thefree radically polymerized material and droplets of the cationicallypolymerized material from a 3D printer nozzle.
 12. The polishing pad ofclaim 9, wherein the polishing pad is annealed by exposure to radiation.